Cerebrum (Telencephalon) and Its Function

The human brain has undergone the greatest transformation during phylogeny, and is, therefore, the “most human” part of the central nervous system; this reorganized the “older” parts of the nerve axis, which is called “encephalization” and provided specific human activities and behavior.

This part of our body still amazes scientists and researchers. One of especially interesting parts is the cerebrum or telencephalon. The so-called large brain consists of the older part, the diencephalon, and the younger part, telencephalon (1). The thalamus and hypothalamus are located in the area of the diencephalon.

The thalamus is a complex nucleus of “interlocking” of the sensory information on their journey to the cerebral cortex. It supplies specific areas of the cerebral cortex (“centers” for touch, deep sensation, orientation in space, etc.) with information necessary for conscious experience.

Moreover, it modulates primary sensory information important for selective sensory experiences and the regulation of consciousness. The hypothalamus plays an important role in the regulation of vegetative functions, as it mediates between brain functions, the internal secretion system, and the autonomic nervous system (2).

Structure of Cerebrum

The large brain, as sometimes referred to, anatomically, consists of two cerebral hemispheres. They are centrally connected by thick bundles of white brain substance, where the transverse fibers of the “corpus callosum” and subcortical associative fibers are located.

These connect further parts of the cerebral cortex and with phylogenetically older subcortical formations: thalamus and basal ganglia.

The surface of the cerebrum, i.e. its core is abundantly furrowed so that by numerous shallower and deeper furrows (sulci) it is divided into a large number of curls (gyri). Among the deep fissures, the most prominent is the lateral Sylvian fissure, which lies between the frontal and temporal lobe.

In this way, the total area of the human cerebral cortex is increased to about 2000 cm², without increasing the volume of the brain or skull volume (2).

Simply put, the cerebrum consists of three key segments. Those are the cortex, i.e. the outer surface where the neuron bodies – gray matter is located; white mass, the bundles of myelin neural fibers; and the subcortical structures that emerged into the white mass. Those are the basal ganglia, the amygdala, and the hippocampus (2).

The cortex is the outer part of the brain surface that is 1.5 to 4.5 mm thick. It represents the gray brain substance, with 12 to 18 billion neurons. Its area approximately measures 2300 cm2. At the same time, this is the most complicated part of the nervous system.

The parts of the cortex of the frontal lobe are phylogenetically “youngest” and are characteristic for humans, and the parts of the islands (insulae) and limbic lobes are the oldest.

According to the cellular structure (cytoarchitectonic distribution, according to the German neuroanatomist Korbinian Brodmann / 1868–1918 /), the cerebral cortex can be divided into 11 areas and 43 fields; the oldest parts of the cortex (paleocortex) have different number of layers, while the younger part of the cortex (neocortex) is uniformly made up of six layers, but differently arranged according to functions: sensory, motor and associative.

According to function, the cerebrum or telencephalon, i.e. its every hemisphere is divided into 5 to 6 lobes. Those are the frontal lobe, parietal (temporal) lobe, occipital lobe, then comes the insula, and marginal lobe or limbic system.

Cerebrum cortex

As we already know, the brain consists of nerve cells (neurons) and supporting cells (glia). Neurons are connected by axions (they transmit signals to other neurons) and dendrites (which receive signals from other neurons), and glial cells make up the majority of the brain mass – their role has not yet been fully explored, they do not transmit signals, but there is increasing evidence that they play an important role in the development of the nervous system.

The cerebral cortex (cortex cerebri) is divided into fields with specific functions such as sight, hearing, smell, and sensation, and controls higher functions such as speech, thinking, and memory. The most important part of the brain related to self-development techniques is the anterior cortex, the one that is located in the front (2). 

In humans, most of the cortex is occupied by the neocortex (isocortex), which is made up of six layers:

  • Lamina molecularis, which is a superficial layer containing very few nerve and glial cells.
  • Lamina granularis external is the second layer dominated by small circular (granular) nerve cells.
  • Lamina pyramidalis external is the third layer, with pyramidal cells which are the characteristic of the cortex of the big brain or cerebrum. In this layer, pyramidal cells can reach up to 40mm in size.
  • Lamina granularis internal is the fourth layer, with many granular cells.
  • Lamina pyramidalis internal is the fifth layer containing very large (Betz) pyramidal cells.
  • Lamina multiformis is the deepest layer of cortex of the brain where we find sparsely distributed neurons of irregular shape (2).

The pyramidal layers are better developed in the areas of the motor cortex, and there are also the largest pyramidal cells (Betz cells) that can reach up to 120 microns in size. In contrast, granular layers are better developed in the sensory areas.

The layers of the cortex are intertwined one with another without sharp boundaries, and the lamina multiformis also turns gradually into the white matter.

Brain Morphology – The Frontal Lobe

The frontal lobe of the cerebrum extends from the front pole to the central fissure. It is possible to identify the precentral, upper forehead and lower forehead fissure. These fissures form four curves: precentral, upper, middle, and lower.

It is in the precentral fissure where the motoric field is located. There is an auxiliary motoric field in the upper front fissure too. There is an ocular field in the posterior 1/3 of the midline.

The lower end of the fissure contains the frontal and frontoparietal portions of the operculum. In the aforementioned fissure of the dominant hemisphere, one can identify the Broca’s area that is responsible for speech.

Brain Morphology – The Temporal Lobe

The temporal lobe extends from the temporal pole to the occipital lobe and we can identify the following segments on this lobe: upper and lower temporal lobe and three curves: upper, middle and lower curve.

The upper temporal lobe extends into the supramarginal vortex. It builds the temporal lobe (gyrus angularis). It contains the main hearing centers and Wernicke’s speech center.

The median temporal curvature is located below the upper curvature, and, in the back, it is extended by the angular curvature. The lower temples are below the middle temple. In the front, all three curves merge in the region of the temporal pole.

Brain Morphology – The Parietal Lobe

The parietal lobe is located behind the frontal lobe in the front of the occipital lobe and above the temporal lobe. It has two fissures or grooves that create one twist, the upper and lower parietal lobes.

In the postcentral fissure, there is a major sensory field. The upper parietal lobe contains associative areas involved in somatosensory functions. The lower parietal lobes are located below the upper parietal lobes.

Its front part is shaped like a forceps and is called supramarginal bend (gyrus supramarginalis). The middle part builds an angular swirl (gyrus angularis). The supramarginal swirl connects three input systems: somatosensory, visual, and auditory system.

 With the postcentral fissure, it builds the vertex part of the lid (operculum parietale). The angular swirl is located at the point where the parietal, occipital, and parietal lobes touch. There is an associative vision area at this point.

Brain Morphology – The Occipital Lobe

The area occupied by this lobe extends from the occipital pole to the parieto-occipital notch. Its key function is processing visual information.

Island

The island (lobus insularis) builds the floor of the outer fissure. It is surrounded by a circular fissure and is covered by parts of the adjacent lobes that build the lid (operculum). On the outside, one can note the sill (tin), shorter, and longer curves. The deep side of the island continues to the putamen and the cloister.

Conclusion

The cerebrum or telencephalon is the largest portion of the human brain. It consists of three key segments, the cortex, the white matter, and the subcortical structures. Also, it has a well-known segmentation to two hemispheres, the right and the left one.

Corpus callosum connects them and makes the communication between the two spheres possible.

The cerebrum has key roles in interpreting the vision. Also, it interprets hearing and touch. It also plays an important part in performing voluntary movements. Movement control, emotions, and learning are under the control of this part of our brain.

Cortex is the surface of the cerebrum and it is peculiar for its hills and valleys. It contains around 16 billion neurons.

References:

  1. Netter H. F. Atlas of human anatomy, Fourth edition, Saunders Elsevier, 2006. Found online at: https://www.scribd.com/document/347005739/Atlas-of-Human-Anatomy-4th-ed-by-Frank-H-Netter-pdf
  2. Ackerman S. Discovering the Brain. Washington (DC): National Academies Press (US); 1992. 2, Major Structures and Functions of the Brain. Available from: https://www.ncbi.nlm.nih.gov/books/NBK234157/